1. Field of the Invention
This invention relates to a power transmission apparatus of the hydraulic pump type for transmitting torque in accordance with a hydraulic pressure which is produced in response to a difference in rotational speed between two rotary shafts, and more particularly to a power transmission apparatus of the type mentioned which is suitable for use with a four wheel drive apparatus of an automobile.
2. Description of the Prior Art
In a four wheel drive apparatus, for example, wherein front and rear wheels are driven commonly by a single engine, the front and rear wheels sometimes rotate at different rotational speeds because they have somewhat different effective rotational radii or because rolling routes are different not only between the left and right wheels but also between the front and rear wheels when the automobile makes a turn or the like. Such a difference in rotational speed between the front and rear wheels will cause an undesirable tight corner braking phenomenon.
Automobiles of the so-called full time four wheel drive type have been developed, therefore, which include a power transmission apparatus interposed between front and rear wheels for preventing occurrence of a tight corner braking phenomenon and for distributing driving force of an engine to the front and rear wheels in response to a difference in rotational speed between the front and rear wheels.
Such a power transmission apparatus either includes a differential gear (center differential a gear interposed between front and rear wheels for absorbing a difference in rotational speed between the front and rear wheels and a differential locking mechanism for looking operation of the front or rear wheels (or a viscous coupling for limiting operation of the front or rear wheels), or singly includes a viscous coupling.
A power transmission apparatus of the type which includes a center differential gear is complicated in structure because such a center differential gear cannot readily be reduced in size and will cause an increase in weight of a car body and also in production cost and besides a differential looking mechanism, a viscous coupling or a like mechanism for assuring the four wheel drivability is required
To the contrary, a power transmission apparatus of the type which singly includes a viscous coupling has such a torque transmission characteristic that the torque transmission gradually decreases as the difference in rotational speed between front and rear wheels increases. Accordingly, when the automobile travels on a road having a very low coefficient of friction such as, for example, a sandy road a muddy place or a snowy or slushy road, if the difference in rotational speed between the front and rear wheels is not sufficiently great, torque cannot be transmitted to the driven side wheels, and consequently, there is the possibility that a slip may occur at the driving side wheels.
A novel power transmission apparatus has been thus proposed and is disclosed, for example in British patent No. 2,164,522 and U.S. Pat. No. 4 676,336 wherein torque is suitably transmitted from the front wheel side to the rear wheel side by means of a hydraulic pressure mechanism in place of such a center differential gear and a viscous coupling as described above.
The power transmission apparatus includes a hydraulic pump interposed, for example, in a driving system which interconnects front and rear wheels. The hydraulic pump includes a rotor connected to be rotated in response to one of front and rear wheels, and a casing connected to be rotated in response to the other of the front and rear wheels. The rotor is accommodated in a coaxial relationship in the casing such that the former may rotate relative to the latter at a rotational speed corresponding to a difference in rotational speed which occurs between the front and rear wheels. The hydraulic pump thus accomplishes transmission of driving force between the rotor and the casing thereof, that is between the front and rear wheels of the automobile by way of a hydraulic pressure which is produced in the hydraulic pump in response to relative rotation between the rotor and the casing. The power transmission apparatus has excellent features, comparing with a power transmission apparatus which employs a viscous coupling, that great driving force can be transmitted with a compact construction and that occurrence of the tight corner braking phenomenon can be restricted significantly.
With the conventional power transmission apparatus described above, however, mission oil is employed as working oil because the apparatus is installed in the inside of a transmission case. Such mission oil frequently contains iron powder produced in the inside of the transmission case or some other dust and so on, which has a bad influence on the power transmission apparatus.
It may thus seem recommendable to provide a power transmission apparatus independently of a transmission case and use particular oil exclusively as working oil. Where the working oil is enclosed in the apparatus, the power transmission apparatus can be formed as a unitary block and can thus be mounted at various locations.
With such a power transmission apparatus as described just above, however, the working oil presents a wide range of variation in temperature whether it is operating or at rest, and the temperature of the working oil is varied also by a temperature of the external air. Such temperature variation will cause a variation in volume of the working oil. For example, when the temperature rises, he working oil is expanded to raise the pressure of the working oil, but on the contrary when the temperature drops, the working oil is contracted to lower the working oil pressure.
Accordingly, where the working oil is enclosed in the inside of the apparatus, a rise of temperature of the working oil to a high temperature may cause spouting of the working oil through a sealing portion or the like of the apparatus, and a drop of temperature to a low temperature may cause the external air to be introduced into the apparatus through a sealing portion or the like of the apparatus, which may lead to a trouble of the apparatus.
Therefore, some means is required for permitting a variation in pressure of working oil.
Such means for permitting a pressure variation of enclosed working oil is disclosed, for example, in U.S Pat. No. 3,393,583.
The disclosed apparatus is designed to permit a pressure variation involved in variation in volume such as expansion or contraction of working oil enclosed in a differential gear of an automobile. More particularly, it is proposed to provide a diaphragm or a pneumatic piston in a working oil chamber.
By the way, when the apparatus is operating, a rotary element in a working oil chamber such as a shaft or a gear wheel, or else, in such a power transmission apparatus as described just above, the entire apparatus rotates to provide centrifugal force to the working oil. Consequently, the pressure of the working oil rises near an outer periphery of the inside of the apparatus but lowers at or near an axis of rotation of the apparatus.
In the apparatus disclosed in U.S. Pat. No 3,393,583 mentioned above, however, since the volume variation permitting means is located near an outer periphery of a working oil chamber, it will act in such a manner as to absorb the oil pressure raised by centrifugal force. Consequently, if the volume of the working oil is varied by a rise of the oil temperature after then, the variation in volume may not fully be absorbed by the volume variation permitting means. Besides, it is hard to compensate for a drop of pressure at or near the axis of rotation.
Thus, for a power transmission apparatus wherein torque is transmitted by way of a hydraulic pressure which is caused by a difference in rotational speed between two rotary shafts, it is a subject how to prevent reduction in pressure of working fluid at or near an axis of rotation while a variation in volume of the working oil caused by a variation in temperature and so on is fully absorbed.